Light emitting devices (LEDs) emit light in response to excitation by an electrical current. One typical LED includes a heterostructure grown on a substrate by metal-organic vapor phase epitaxy or similar techniques. A LED heterostructure includes n and p type semiconductor layers that sandwich light producing layers. Exemplary light producing layers may be quantum wells surrounded by barrier layers. Typically, electrical contacts are attached to n and p semiconductor cladding layers. When a forward bias is applied across electrical contacts electrons and holes flow from n and p layers into a light producing active region. Light is produced when these electrons and holes recombine with each other in an active region comprising at least one semiconductor material.
The efficiency with which a LED converts electricity to light is determined by the product of the internal quantum efficiency, the light-extraction efficiency, and losses due to electrical resistance; this product is also termed the external quantum efficiency. The internal quantum efficiency is determined by the quality of the semiconductor layers and the energy band structure of the device. Both of these are determined during deposition of semiconductor layers. The light extraction efficiency is the ratio of the light that leaves the LED chip to the light that is generated within the active layers. The light extraction efficiency is determined by the geometry of the LED, self-absorption of light in semiconductor layers, light absorption by electrical contacts, and light absorption by materials in contact with the LED that are used to mount a device in a package. Semiconductor layers have relatively high indexes of refraction; consequently, most light that is generated in an active region is internally-reflected by surfaces of a chip many times before it escapes. To achieve high light-extraction efficiency it is important to minimize absorption of light by the semiconductor layers and by electrical connections to the chip. When these layers are made to have very low optical absorption, by being transparent or highly reflective, the overall light extraction in an LED is improved substantially.
When an LED is energized, light emits from its active layer in all directions, impinging on the LED surfaces at many different angles. Typical semiconductor materials have a high index of refraction compared to ambient air, n=1.0, or encapsulating epoxy, n≈1.5. According to Snell's law, light traveling from a material having an index of refraction, n1, to a material with a lower index of refraction, n2, at an angle less than a certain critical angle θc relative to the surface normal direction will cross to the lower index region, whereθC=sin−1(n2/n1)  (1)
Light that reaches a semiconductor surface at angles greater than θC will experience total internal reflection. This light is reflected back into the LED chip where it can be absorbed within the chip or in metal contact layers that are attached to the chip. For conventional LEDs, the vast majority of light generated within the structure suffers total internal reflection at least once before escaping from a semiconductor chip. In the case of conventional GaN-based LEDs on sapphire substrates about 70% of emitted light is trapped between the sapphire substrate and the outer surface of the GaN. This light is repeatedly reflected, greatly increasing its chance for reabsorption and loss.
Several prior art approaches have been used to create reflective ohmic contacts for LEDs. The simplest is to use a thick sheet or layer of metal that has a high reflectivity. These include Al, Ag, Rh, Pd, Cu, Au, Cr, Ti, Ni, nickel/gold alloys, chrome/gold alloys, silver/aluminum mixtures, combinations of the preceding and others known to one familiar with the art. The chosen metal needs to not only have a high reflectance, but also make a low resistance ohmic contact. In the case of p-type AlInGaN, only Ag combines low electrical resistance with high reflectivity. Unfortunately, Ag presents a reliability concern because it is subject to tarnish and it is subject to electromigration during device operation. Also, the contact resistance of Ag-based contacts sometimes increases with time during device operation.
Wierer, et al., WO 01/47038 and U.S. Pat. No. 6,992,334, disclosed a multi-layer contact with one metal to make the ohmic connection to the device and a second metal to provide high reflectivity. The first metal may be a low reflectance metal because it is kept very thin, less than 20 nm, so that light penetrates through to the more reflective layer. The more reflective metal may be very thick to spread current effectively across the chip. The reflectivity of the combined metal stack can exceed 75%. However, these contacts are still too absorptive to achieve very high light-extraction because the reflectance of metals is limited to below about 93%; in addition, metals suitable for forming an ohmic contact are highly absorbing.
Schubert in U.S. Pat. No. 6,784,462 and in Applied Physics Letters, 84, 22, 31 May 2004, 4508, disclosed a reflective submount for an LED that interposed a dielectric layer under a portion of the metal contact to improve the overall reflectivity. In this scheme a plurality of ohmic contacts are made to the semiconductor using metal vias through the dielectric layer. The metal in these vias makes contact to spots on the semiconductor and a semiconductor layer spreads current between the spots. Schubert teaches a dielectric with a thickness chosen to be about one-quarter of a wavelength. This single dielectric layer serves to improve the reflectance compared to a simple metal reflector, but it does not provide for high reflectance for light incident at high angles, since the dielectric is so thin. The contact area to the semiconductor is small for these contacts which can significantly increase the resistance to electrical current flow, reducing the overall efficiency of the LED. Also, since the electrical contact is made in spots the current injection is not uniform over the surface of the chip. In the Applied Physics publication Schubert teaches that the “ . . . micro-contact array covers only about 2% of the entire back side lit area of the LED chip.” Schubert in Applied Physics Letters, 87, 031111 (2005), discloses an “omni-directional reflector” realized by the combination of total internal reflection using a low-refractive-index material and reflection from a one-dimensional photonic crystal. The low-refractive-index material is nanoporous SiO2 with an index of refraction about 1.1; Schubert notes that n must be below 1.112 or the reflectivity falls off sharply. The one-dimensional photonic crystal structure is achieved with four pairs of very thin films of TiO2 and SiO2 with a very narrow tolerance on each film thickness. Schubert's data indicates quite good reflectivity; however the process he teaches is complex and has quite tight tolerances requiring very expensive process controls; low yields can be anticipated.
Dielectric Bragg reflectors, DBRs, have been disclosed in U.S. Pat. No. 6,552,369 wherein an epitaxially grown AlGaAs/AlInGaP structure is taught. In U.S. Pat. No. 5,585,648 the device comprises a SiC substrate with an optional DBR made from AlInGaN. U.S. Pat. No. 6,492,661 teaches a refection layer with a current blocking region; U.S. Pat. No. 6,492,661 also teaches substitution of an alternative substrate for an original substrate after device fabrication. U.S. Pat. No. 6,797,987 teaches a transparent, electrically-conductive oxide layer over coated with reflective layer; a substitute substrate replaces the original substrate; no oxide layer or Bragg reflector layer is taught.
Therefore a need exists for a reflector structure that provides for low-resistance contacts and conduction across a device while at the same time providing for high reflectance of light incident at all angles and can be manufactured in high volumes at low cost.